This invention relates to a propulsion system for a marine vessel and more particularly to an improved counter-rotating propeller arrangement for such vessels.
It has been proposed to employ a propulsion system for watercraft that utilizes a pair of counter-rotating propellers that are disposed with a common rotational axis and one in front of the other. By using blades having a pitch of opposite hands, this dual propeller arrangement can provide significant improvements in propulsion efficiency. When the propulsion unit is provided with a forward neutral reverse transmission, the two propellers are both driven in opposite directions during a forward drive mode. Only one of the propellers is normally driven in reverse since the watercraft is normally not propelled at such large powers or at such high speeds in reverse.
Although this propulsion arrangement has a number of advantages, there are some areas where the performance can be improved. Although the dual propellers provides improved propulsion efficiency, when accelerating from a low speed to cruising or high speed conditions, the drag of the two propellers is significant enough to reduce the ability of the engine to accelerate as rapidly as desirable. As a result, these systems may at times provide less than maximum acceleration capabilities.
It is, therefore, a principal object of this invention to provide an improved dual propeller driving arrangement for a vessel.
It is a further object of this invention to provide an improved dual propeller assembly wherein the drag of at least one of the propellers is reduced at least on acceleration conditions so as to allow the engine and watercraft to accelerate more rapidly.